Use of powered tools to drive threaded fasteners at high speed and high torque loads results in high forces applied by the driver to the fastener. Although many threaded fastener drive systems, particularly those with a driver-engageable recess in the fastener head, are designed to have surfaces that are engaged by corresponding surfaces on the driver, such ideal surface-to-surface engagement, at best, is difficult to achieve in practice. Rather than surface-to-surface engagement between the driver and fastener, by which the driving load can be distributed over a broad surface area, driver-recess engagement often is concentrated in small areas or points. That may result from a number of factors such as inconsistencies in the manufacture of the fastener or the driver, as well as difficulties encountered in the field. Field-encountered difficulties may include, for example, misalignment of the driver and fastener or inability to fully seat the driver in the recess because of paint or other debris that may have collected in the recess. Even slight misalignment between the driver and the fastener, or a variation of the fastener or driver from design specifications, can result in substantial reduction in the area of contact between the driver and fastener, in many cases resulting in near point-like contact of several portions of the driver and fastener. Application of high torque under such circumstances necessarily results in concentrated stresses in the materials of the driver and the recess that, in turn, can lead to failure of the material, either by plastic deformation or fracture. Even slight plastic deformation of the engagement surfaces of the recess and driver can adversely affect system performance. If the recess deforms to define ramp-like surfaces inclined from the vertical, the driver may "cam-out" of the recess under the influence of the applied load. Such cam-out is undesirable, not only because it results in premature or uncontrollable disengagement of the driver and recess, but also because the suddenly disengaged driver can slip onto and damage the work piece. Additionally, excessive stress in the driver blade while driving the fastener can cause the blade to deform in a manner that reduces the surface area contact with the fastener and effectively shifts the region of contact radially inwardly, thereby reducing the effectiveness of driver-recess engagement and increasing the risk of failure. Even when the sidewalls of the driver and recess wings engage in broad surface-to-surface contact, the point at which the resultant force applied by the driver to the recess sidewall typically is at a center of effort located generally at the center region of the sidewall. Consequently, the resultant force is applied at a location that is substantially radially inwardly from the outermost extremity of the sidewall. The foregoing difficulties may be encountered whether the fastener is being driven in an installation or a removal direction. In many applications, the ability to remove the fastener quickly and effectively is at least as, if not more, critical than its installation.
A number of recess and driver engagement systems have been developed to enhance the efficiency of the drive system, to reduce the risk of cam-out as well as to improve other aspects of the drive system. One such system that has had use in aircraft applications is described in U.S. Pat. No. Re 24,878 (Smith et al.). The recesses are defined by three or four wings that extend radially from the central portion of the recess. Each wing defines an installation wall and a removal wall, both of which are to be substantially vertical, that is, to lie in a plane that parallels the central axis of the fastener. The driver has a complementary configuration. The system is intended to promote axial alignment of the driver and fastener by eliminating the tendency for the driver to "rock" in the recess as well as to resist cam-out. Axial alignment of and full seating of the driver within the recess is essential to obtain the benefit of the vertically oriented driving and removal walls. Even with this system, application of high torque loads may cause some deformation of the wings of the driver in a manner that tends to reduce the area of the region of contact between the driver blades and the walls of the recess wings as well as to shift the contact region radially inwardly. That, in turn, increases the risk of plastic distortion of the recess that can lead to progressive deterioration of the driver-recess engagement with resulting adverse consequences.
Another driver-recess engagement system is described in U.S. Pat. No. 3,237,506 (Muenchinger) that has been incorporated in fasteners commercially developed for commercial application under the trade designation Pozidriv.RTM.. Among the characteristics of the Pozidriv.RTM. system is that the sidewalls of each of the wings of the recess is designed to lie in a plane that is substantially vertical. In forming such substantially vertical sidewalls by the conventional cold header technique in which a two-blow heading machine impacts the end of the wire or other material from which the fastener is made, while the wire is supported in a die of the heading machine, first with a punch that forms a bloom (a partially formed head) on the end of the fastener blank and then with a finishing punch that finishes the head and forms the driver-engageable recess. The operation is carried out automatically and at high speed. The punches are impacted against and withdrawn from the head end of the fastener blank along the longitudinal axis of the fastener blank. Among the constraints inherent in the heading process is that the recess design should be free of undercut regions, that is, regions that, although being formable as the punch is impacted into the fastener head, would be obliterated when the punch is retracted. The Muenchinger patent describes a recess and punch configuration intended to reduce or eliminate the effect of "metal fall-away" that tends to occur when punching a recess in a fastener head. The result of the phenomenon of metal fall-away is that the recess does not conform precisely to the configuration of the recess-forming punch. The lack of accurate conformance results in a recess that will exhibit increased cam-out and will reduce driver stability, resulting in a loose, wobbly fit between the driver and the recess. Those difficulties result in a reduced torque capacity of the mated fastener and driver.
U.S. Pat. No. 4,187,892 (Simmons) and U.S. Pat. No. 5,120,173 (Grady) describe a drive system for threaded fasteners in which intentionally deformable ribs are provided on two or more of the driver-engageable sidewalls of one or both of the recess and driver. The ribs project slightly from the sidewalls and are designed to deform or cause deformation of the ribs they engage in a manner that provides an interlocking engagement. The interlocking engagement of the ribs resists cam-out. Such anti-cam-out ribs may be incorporated in recesses having substantially vertical drive walls as well as those recesses in which the sidewalls are inclined substantially from the vertical. The ribs are formed during the cold heading process in which the recessed fastener head is formed in a two-blow heading machine.
Although the inclusion of anti-cam-out ribs on the recessed head fasteners significantly improves the drive performance of the fasteners, some types of recessed head fasteners may require compromises in the design in order to include the advantages of the anti-cam-out ribs. The anti-cam-out ribs cannot be formed in a manner that would leave an undercut that would result in the rib being torn out as the recess forming punch is retracted. In recesses with substantially vertical sidewalls the anti-cam-out rib also must be essentially vertical and parallel to the fastener axis. Although it would be desirable to locate such ribs at a maximum radial distance from the central axis of the fastener, that is, at the more radially outward regions of the recess sidewalls, the height of the recess sidewalls progressively decrease in height toward their radial extremities. Consequently, the vertical height of the rib necessarily is very short toward the radial extremities. Such a rib can only be engaged by a ribbed driver blade near the upper end of the recess adjacent the top surface of the fastener head. A short rib, so located, presents greater risk of improper engagement or non-engagement by the ribbed driver. Consequently, an anti-cam-out rib, particularly in a recess having vertical or near vertical drive walls, typically has been located more radially inwardly along the sidewall.
Also among the compromises to be considered when employing a ribbed recess is that the protrusion of the rib into the envelope of the recess wing necessarily requires either that the width of the wing be increased or that the width of the mating driver blade be reduced to accommodate that projection. That, in turn, requires a reduction in the mass of material of the fastener head, or the driver blades, or both, in order that the driver blade can be properly inserted into the recess. This design compromise may be particularly pronounced in those applications where it is desirable to provide an anti-cam-out rib on both the installation and the removal sidewalls of the recess wings. Additional complications result from the desirability that such recesses should be compatible with existing drivers. In many cases, the inclusion of anti-cam-out ribs in the recess may restrict the extent to which an existing driver can penetrate into the recess, possibly preventing full depth penetration, consequently reducing the effectiveness of torque transmission.
The anti-cam-out ribs that have been incorporated into recessed head fastener systems typically have had a V-shaped cross-sectional profile that defines a relatively sharp apex along the length of the rib. When such a fastener is intended for use in an environment where a plating will enhance its function (e.g., an anti-corrosive plating), the very small area of intersection of the driver and recess ribs can be expected to result in high stresses sufficient to fracture the plating, particularly when the driver also has ribs on its blades.
It would be desirable to provide improvements in recessed head fasteners and drivers by which the foregoing and other difficulties are reduced or eliminated.